A process for producing a porous film of an aromatic polyamide (hereinafter referred to as an "aramid", sometimes) is described in Japanese Patent Kokoku Nos. 59-14494 and 59-36939. In the former, there is suggested a process comprising casting a solution prepared by dissolving an aramid in a solvent in the form of a film, and solidifying the film and removing the solvent from the solidifyed film to produce a porous film. In the process, it is explained that an uniform porous film having a pore diameter of several angstrom to 100 .mu.m can be produced because a polymer is not agglomerated and precipitated. In the latter, there is described a process of producing a porous film from a coacervation composition prepared by adding a poor solvent to an aramid solution. It is explained that an uniform porous film can be produced with causing no difference in porosity between the surface and interior of the film because the coacervation is composed of a liquid phase enriched with a colloid and a liquid phase which is lacking in the colloid.
In Japanese Patent Kokai (Laid-Open) No. 2-222430, there is described a microporous aramid film wherein micropores exist continuously on both sides of the film. It is explained that the film is superior to the above-described porous film in heat resistance and is superior in dimensional stability to humidity. In comparison with Japanese Patent Kokoku No. 59-36939, the process of the film is characterized by including a step of stretching a casted film in a longitudinal direction immediately after immersing it in water, i.e. step of stretching.
However, the porous film obtained by the process described in the above gazettes is a film in the form having penetrated pores, as is apparent from the description of Japanese Patent Kokai (Laid-Open) No. 2-222430, i.e. description that the aramid porous film has continuous micropores. Also, the porous film does not form a fibril in the form of a network or a nonwoven fabric in the present invention, as is apparent from the Comparative Examples described hereinafter.
It is explained in the above-described Japanese Patent Kokai Nos. 59-14494 and 59-36939 that the resultant porous film is useful for a battery separator. Furthermore, in Japanese Patent Kokai (Laid-Open) Nos. 53-58636 and 5-335005, there is a description about a battery separator of an aramid.
In Japanese Patent Kokai (Laid-Open) No. 53-58636, there is described that it becomes possible to reduce deterioration under high temperature, stabilize characteristics of a battery and prolong a lifetime by using a fabric or an nonwoven fabric of an aramid as a battery separator. In Japanese Patent Kokai (Laid-Open) No. 5-335005, there is described that a nonwoven fabric of an aramid fiber or a porous film, more specifically a NOMEX.RTM. paper (meta-aramid paper) manufactured by Du Pont Co. is used as a separator of a lithium secondary battery.
Under these circumstances, one objective of the present invention is to provide a para-aramid porous film having an uniformity and fine vacant spaces, which can not be accomplished by a nonwoven fabric, and a process for producing the same, by making use of characteristics of a para-aramid (e.g. high heat resistance, high rigidity, high strength, excellent solvent resistance, etc.). Another objective of the present invention is to provide a battery separator using the para-aramid porous film.